1. Field of the Invention
This invention relates a process for preparing an attrition resistant catalyst. More specifically, but not by way of limitation, the invention relates to a method of imparting attrition resistance to a particulate catalyst or catalyst precursor (e.g., vanadium/phosphorus oxide, V/P/O, solids) by spray drying a slurry containing catalyst or catalyst precursor and optionally a large size silica colloidal sol, polysilicic acid or mixtures thereof in the presence of H3PO4 followed by calcining the spray dried product.
2. Description of the Related Art
The use of inert metal oxides such as silica or the like as a support for catalysts or as a binder for catalyst particles is generally well known in the art. In particular, U.S. Pat. No. 4,677,084 discloses and claims a process for producing attrition resistant catalyst or catalyst support particles having an oxide-rich surface layer as well as claims the attrition resistant particles. In a divisional U.S. Pat. No. 4,769,477 the use of attrition resistant vanadium/phosphorus oxide catalyst particles (made by the above process) having a SiO2-rich surface layer for producing maleic anhydride by oxidation of a hydrocarbon is similarly disclosed and claimed. The method of producing the attrition resistant catalyst particles according to these patents involves the forming of a slurry of catalyst, catalyst precursor or catalyst support particles in a solution containing a solute consisting essentially of the oxide precursor particle (i.e., that which becomes or forms the resulting oxide-rich surface layer; e.g., silicic acid, polysilicic acid or the like). This oxide precursor particle useful in this process is characterized by an average particle size no greater than 5 nanometers and the relative amount of the same is chosen such that the weight of the oxide to be formed as a surface layer is about 3 to 15 percent of the total weight of the resulting attrition resistant particles. This slurry is then spray dried to form porous microspheres which are then calcined to produce attrition resistant catalyst. Both of these patents further contain extensive background reviews of relevant prior patent references and the present invention can be viewed as a further improvement relative to each. As such, U.S. Pat. Nos. 4,677,084 and 4,769,477 are incorporated herein by reference for disclosure purposes.
U.S. Pat. No. 5,302,566 discloses an alternate method for preparing an attrition resistant catalyst described above, particularly vanadium/phosphorus oxide catalyst, having an oxide-rich surface layer. In this alternative method the slurry to be spray dried comprises the catalyst or catalyst precursor particles and a mixture of a solution containing a solute consisting essentially of the oxide precursor particles of less than 5 nm along with a colloidal oxide sol wherein the oxide sol particles have an average size of between 5 and 7 nm. The amount of colloidal oxide sol is selected such as to provide between 50 to 95 percent by weight of the final oxide-rich surface and the polysilicic acid provides 50 to 5 percent. Again, the oxide-rich surface is between 3 and 15 percent by weight of the total weight of resulting attrition resistant catalyst. The resulting catalyst made by this alternative process is shown to be comparable in attrition resistance properties to a catalyst made using only oxide precursor solution as the surface forming oxide source. The mixture of combined oxide precursor solution and 5 to 7 nm colloidal sol, however, is significantly more stable than a solution of oxide precursor alone. Consequently, advantages in terms of shelf-life, storage, and handling are realized particularly when scaling up to commercial production levels. U.S. Pat. No. 5,302,566 is incorporated herein by reference for disclosure purposes.
In a copending and commonly assigned U.S. patent application Ser. No. 09/163,680 filed Sep. 30, 1998, now U.S. Pat. No. 6,107,238, incorporated herein by reference, a further improvement relating to the above described processes is disclosed. In this process the colloidal oxide sol employed has an average size between 10 and 100 nm and the amount used is selected such that from 25 to 50 percent of the resulting weight of attrition resistant catalyst is derived from the colloidal oxide sol. The soluble solute component (e.g., the silicic acid or polysilicic acid) in the slurry prior to spray drying again is characterized by an average particle size no greater than 5 nm and the amount employed is selected such that from 5 to 15 percent of the weight of the attrition resistant catalyst (including the colloidal sol contribution) is derived from the soluble oxide precursor. This particular process and resulting attrition resistant catalyst is intended to alleviate a specific problem associated with transition metal oxide containing catalysts that can expand and shrink during the oxidation and reduction cycles associated with continuous use and the associated increase attrition losses observed during the reduced state.